1. Field of the Invention
The present invention relates to a sub-mirror unit for use in a camera. More specifically, the present invention pertains to a single-lens reflex camera featuring an integrally-formed sub-mirror and a method for making the sub-mirror.
2. Description of the Related Art
Initially, a prior art sub-mirror unit 3 for use in a single-lens reflex camera will be described with reference to FIGS. 5 and 6.
In these figures, reference character 3c denotes a sub-mirror and reference character 3a denotes a sub-mirror supporting plate for fixedly supporting the sub-mirror 3c.
The sub-mirror 3c is formed of a glass plate on the surface of which a material is evaporated to form a reflective layer. The sub-mirror 3c is mounted such that its surface including the reflective layer faces an object lens of the camera, and is fixedly bonded at its rear surface to the sub-mirror supporting plate 3a by means of a double-coated adhesive tape 3b.
The above-described, prior art sub-mirror unit 3 has the following disadvantages:
(1) Since the sub-mirror unit 3 requires a number of parts and hence a number of assembly steps, the parts and assembly costs are high.
(2) The sub-mirror 3a is prone to mounting angle variations or detachment due to a failure in adhesion or peeling-off of the sub-mirror 3a from the supporting plate 3b.
(3) Since cracks and/or chips are created in the glass edges when a glass plate is cut into a predetermined size, it is necessary to allow a larger margin between an effective mirror area and the glass edges, or to grind the glass edges. However, if the distance between the effective mirror area and the glass edges is increased, the sub-mirror 3c must be made larger in size contrary to the goal of reducing the size of optical equipment, such as cameras. Also, if the glass edges are ground, the manufacturing cost of the sub-mirror 3a would be significantly increased.
(4) Because the glass sub-mirror is bonded to a non-glass material sub-mirror supporting plate 3a, variations in the ambient temperature tend to produce distortions in both structures and/or cause variations in the mounting angle and/or result in peeling-off of the sub-mirror 3a from the supporting plate 3b due to a difference in their respective coefficients of thermal expansion.
FIG. 14 is a sectional view showing a layout of the primary components of a prior art single-lens reflex camera having an automatic focus adjusting device. An object lens (not shown) is attached to a mount 101, and light passing through the object lens is reflected by a main mirror unit 102 upwardly to a focusing plate 106. More specifically, a portion of the light from the object lens passes through the main mirror unit 102 and, after being reflected by a sub-mirror unit 103, is guided to a focus detector 104 positioned under the sub-mirror unit 103. A photographer observes an image of the object formed on the focusing plate 106 through a pentaprism 107 and an eyepiece lens 108.
The sub-mirror unit 103 is constructed in the same manner as the sub-mirror unit 3 as described above with reference to FIG. 6.
As previously discussed, since small cracks and/or chips are created in the glass edges when a glass plate is cut into a predetermined size, it is necessary to allow a larger margin between an effective mirror area and the glass edges, or to grind the glass edges. If the margin .delta. between the effective mirror area and the glass edges is set to be large as shown in FIG. 14, the light guided to the focus detector 104 would be narrowed particularly in the vertical, i.e., up and down, direction of the object field, making it difficult to enlarge a focus detecting area in the vertical direction of the object field. If the size of the sub-mirror unit 103 is increased, the focus detecting area can be enlarged in the vertical direction of the object field. In practice, however, because the sub-mirror unit 103 is positioned between the main mirror unit 102 and a shutter device 105, it is problematical to increase the size of the sub-mirror unit 103. Also, if the glass edges are ground or cut, the manufacturing costs are significantly increased.
Further, as previously discussed in connection with sub-mirror unit 3, because the glass sub-mirror 3c is bonded to the non-glass metal sub-mirror supporting plate 3a, variations in the ambient temperature tend to produce distortions in both structures, or cause a change in the mounting angle and peeling-off of the sub-mirror 3c from the sub-mirror supporting plate 3a due to a difference in the coefficient of thermal expansion therebetween. These problems increase as the sub-mirror becomes larger. This means that an increase in the size of the sub-mirror 3c due to, e.g., an enlargement of the focus detecting area is not desirable.